This proposal employs a multidisciplinary approach to elucidate how platelets and leukocytes overcome kinetic and mechanical constraints to adhere to vascular surfaces under flow. The focus is the interaction of the three selectins (P-selectin, E- selectin, and L-selectin) with P-selectin glycoprotein ligand-1 (PSGL-1) and other cell- surface glycoconjugate ligands. These interactions mediate rolling adhesion of leukocytes on activated platelets, endothelial cells, and adherent leukocytes. Our overall hypothesis is that important kinetic properties (e.g., catch and slip bonds) for binding of selectins to their ligands result from specific atomic-level interactions that are dictated by the structures of these molecules. Force regulates function by inducing conformational changes and/or forming new atomic-level interactions in the structures. Transport parameters influence how intrinsic docking rates affect molecular interactions. Since these kinetic properties determine cellular function under flow (e.g., tethering, rolling, and aggregation), relatively minor structural differences that alter these atomic-level interactions have major consequences for physiology and pathology. The proposal is integrated into four specific aims. The first three aims use crystal structures, molecular modeling, and biochemical and biophysical assays to define how specific structural features of selectins and their ligands govern tethering, rolling, and aggregation of flowing cells. The fourth aim uses knock-in mice expressing a mutant selectin to reveal the biological functions of flow-enhanced cell adhesion in vivo. The information obtained from this integrated study will clarify how molecular structure fulfills the biophysical requirements for blood cells to adhere in a hydrodynamic environment, and may suggest new therapeutic approaches to inhibiting pathological cell adhesion during inflammation and thrombosis. Project Narrative: In response to infection or injury, circulating white blood cells and platelets adhere to blood vessel surfaces, the first step in controlling infection or bleeding. This project addresses how specific adhesion molecules control this process. This information obtained may offer new methods to treat excessive blood cell adhesion in heart attacks, strokes, deep venous thrombosis, and other disorders.